1. Field of the Invention
This invention relates to D.C. motor speed control apparatus, and more particularly to a D.C. motor speed control apparatus of the type in which the armature winding of the motor is connected in one branch of a Wheatstone bridge circuit with an output proportional to the shaft speed being utilized in controlling the electrical power supply to the motor. Still more particularly, it relates to improvements of an actuating circuit associated with such a motor speed control circuit.
2. Description of the Prior Art
A D.C. motor speed control circuit of the Wheatstone bridge type is, for example, disclosed in Japanese Patent Publication No.Sho 50-40442 (Date of Publication: Dec. 24, 1975). FIG. 1 shows an example of this type of circuit. Here a D.C. motor DCM with an armature winding M having a resistance Ra forms a Wheatstone bridge together with three resistors R.sub.1, R.sub.2 and R.sub.3. Connected across a pair of output terminals or points a and b, is an npn transistor Tr.sub.2 and a spring of two diodes D.sub.1 and D.sub.2. The base of transistor TR.sub.2 is connected to the point a, between registors R.sub.2 and R.sub.3. Its emitter is connected through a resistor R.sub.4 to a negative input terminal or point d between resistor R.sub.3 and the motor armature M. The emitter is also reverse biased by the string of two diodes D.sub.1 from the point b between resistor R.sub.1 and the motor armature. Hence the collector current depending upon the counter-EMF developed across the armature can be utilized to control the electrical power supply to the motor by using a pnp transistor Tr.sub.1. The latter is connected between a positive input terminal c of the Wheatstone bridge circuit and a positive electrode of an electrical power source or battery E through a control switch S.sub.1.
The voltage Et across the motor armature M is composed of two components; one is an equivalent developed counter electromotive force (EMF) Ea and the second is a voltage across the armature resistance Ra, and therefore may be expressed as EQU Et=EA+RaIa (1)
Here Ia is the current flowing through the armature resistance Ra. When the resistance Ra is selected so that the relationship Ra/R.sub.1 =R.sub.3 /R.sub.2 is satisfied, the voltage appearing across the output terminals a and b of the Wheatstone bridge circuit is equal to the counter-EMF developed across the motor armature M. This counter-EMF Ea may be expressed as EQU Ea=(Z/a).multidot.p.multidot..phi..multidot.n (2)
wherein Z is the number of conductors; p is the number of poles; .phi. is the magnetic flux density per pole; a is the number of turns of wire; and n is the number of revolutions of the rotor. All of the above parameters except the n are constants of the design, thus the equation (2) may be written, EQU Ea=Kn (3)
With this Wheatstone bridge circuit, therefore, it is possible to control the number of revolutions of the rotor, that is, the speed of rotation of the motor, as a function of only one variable Ea in a manner such that the output of the Wheatstone bridge circuit is applied to the base of the transistor Tr.sub.1. This can then control the current flowing through the emitter-collector of transistor Tr.sub.1.
In operation, after a stationary state has been attained to drive the motor DCM at a constant speed, the output voltage V.sub.D of the Wheatstone bridge circuit is maintained constant depending upon the difference between the voltage V.sub.F across diodes D.sub.1 and the base-emitter voltage V.sub.EB of transistor Tr.sub.2 regardless of how much current flows through the diodes D.sub.1 and the transistor Tr.sub.2. Assuming that load on the motor DCM is increased a decrease occurs in the number of revolutions of the rotor. The voltage Et across the motor winding is then decreased. This decreases the potential at the point b, along with the emitter potential of transistor Tr.sub.2. This in turn causes an increase in the base potential of transistor Tr.sub.1 relative to the emitter potential. As a result, the current supply to the Wheatstone bridge circuit is increased until the speed of rotation of the motor is restored to the initial predetermined level.
The conventional actuating circuit for use with the D.C. motor speed control circuit of FIG. 1 is shown in the same figure. It includes a resistor R.sub.5 and a capacitor C.sub.1. These are connected in series between the base electrode of transistor Tr.sub.1 and the negative terminal of the direct current supply source E. This actuating circuit is, however, not suited for assurance of initiation of operation of the speed control circuit at the time of closure of the main switch S.sub.1. For example, when the voltage of the energy source E increases at a slow rate to the critical level for the normal operation of the motor DCM, a long time lag results. Alternatively when switch S.sub.1 is opened at the time the motor DCM is restrained, the charge stored on the capacitor C.sub.1 is retained except for spontaneous leakage because of the lack of the discharge circuit therefor. This is so because the transistor Tr.sub.2 is cut off at that time. Consequently, the subsequent closure of switch S.sub.1 before capacitor C.sub.1 is not completely discharged fails to result in actuation of the motor DCM, for transistor Tr.sub.1 is not rendered conductive between the emitter and base thereof.